1. Field of the Disclosure
The invention generally relates to antireflection coatings and to substrates provided with antireflection coatings. In particular, the invention relates to antireflection coatings with high resistance to scratching and other abrasions.
2. Description of Related Art
Antireflection coatings are widely used today for improving the transmission of transparent substrates such as sight-glass windows, or on the other hand for diminishing annoying reflections on the substrate. However, depending on the intended use of the substrate, the antireflection coating might be exposed to high wearing stresses. For example, an outer coating of a vehicle front window, while driving, is exposed to sand and dust particles impinging thereon with a high speed which may result in ablation of the coating over time. A particular stress to such front windows results when a wiper passes over the soiled front window. The sand and dust particles then are not only advanced on the window, but are also forced against the windshield by the rubber lip of the wiper. In this way, long scratches can be formed in the coating.
Scratches and other damage cause turbidity and therefore counteract the purpose of an antireflection coating.
In addition, the driver's vision is impaired.
Therefore there is a need for an antireflection coating with a high resistance to scratching and abrasion.
From US 2005/0074591 A1 a transparent substrate with an abrasion-resistant antireflection coating is known. The antireflection coating is composed of four layers with alternating high and low refractive index. The low refractive index layers comprise silicon oxide (SiO2), the high refractive index layers comprise silicon nitride (Si3N4) or tin oxide (SnO2). The top layer of the layer stack is formed by a low refractive index layer. A disadvantage thereof is that the low refractive index silicon oxide layer is very soft compared to the high refractive index materials, especially compared to Si3N4. So it is just the top layer that still tends to wear quickly. Once the top layer is removed, a high refractive index layer then forms the surface. This results in a reversion of the antireflection effect. The coating now rather acts like a dielectric mirror.
Therefore, an object of the invention is to further improve abrasion resistance of antireflection coatings. Another object of the invention is to provide a layer system which reduces the reflectance of a window pane, in particular of a sight glass, from typically about 4% to less than 1% and at the same time significantly increases the mechanical resistance of the substrate surface.